Well not the naked mole rat, since that does not occur in SA, but rather the Damara mole-rat you mentioned.

And the reason behind this strange social system, is the fact that a large number of individuals are needed to keep the colony functioning. They need to do a lot of tunneling to get enough food to keep the colony going. There is not enough food around to sustain a large number of females that needs extra feeding due to being pregnant. They eat mostly tubers, roots and bulbs. This is not always very nutritional, and it is hard work to get to it.